The origin of this strange word moment will become clear at higher levels as you study the inertia tensor, but when first introduced to it in (I presume) intro physics, stick to the basics of the above equations. It just so happens that $ I $ changes a lot depending on mass, radius, shape, distribution of mass, etc. The moment of inertia is acting as the thing that impedes rotational acceleration, just as mass impedes linear acceleration. Since both $ m $ and $ I $ are a positive constants for an object, they are the constants that control how easy/hard it is to accelerate the translation and rotation of the object, respectively. Generally when teaching a basic physics class I just like to give the following analogous statements of Newton's 2nd Law in simple casesĪnd note that if mass (or intertia, they mean the same thing) is a resistance to a change in linear motion, then moment of intertia is a resistance to a change in rotational motion. I'm assuming this is from the standpoint of first-semester introductory physics, so I don't think it's appropriate at all to talk about tensors or multivariable integration.įorget about what the complicated functional description of $ I $ looks like for some object for just a moment and call it some constant. In practice, it makes no difference, but in principle, it really is a very fundamental distinction. If these theories are correct, then one should, in principle, use the phrase "inertial mass" rather than just "mass" for the analog of "moment of inertia", and "gravitational mass" to describe the way an object is attracted, say, by Earth's gravity. Of course, if it exists, the difference is so small that no experiment on Earth has been able to measure this difference, but it could explain some difficulties with astronomical observations. For these theories, "inertial mass" and "gravitational mass" are not identical. In fact, there are some theories of gravitation (for instance Tensor-Vector-Scalar Gravity) that differ from General Relativity. Postulating that this identity is a fundamental principle of Nature is at the very origin of Einstein's theory of General Relativity. The tendency of an object to gravitationally attract another one and be attracted by it is its "gravitational mass".Įxperimentally, one has always found, since Newton, that "inertial mass" and "gravitational mass" are equal.īut there was no deep reason for this identity. Technically, the analog of "moment of inertia" for translational (your "regular") motion, rather than rotation, is "inertial mass". In fact, your question is deeper than meets the eye. The corresponding technical term for what you call "regular motion" is just "mass". "Moment of inertia" is a precise technical term, related to rotational motion. It refers to a notion that can apply to many things, physical or psychological, like some person being slow to act. Your confusion comes from the fact that the word "inertia" is not a "technical" term.
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